A roadside bomb killed an American soldier Tuesday near the capital of Afghanistan's volatile Helmand province, following the first major US deployment to the southern city since foreign forces withdrew in 2014.

The blast, which occurred during an anti-Taliban operation, left another American and six Afghan soldiers wounded, the US-led NATO coalition said.

The casualties came just a day after NATO announced the deployment of around 100 US troops to Lashkar Gah to help head off a potential Taliban takeover of Helmand's capital as fighting intensifies.

"One US service member died as a result of wounds sustained during operations near Lashkar Gah in Helmand," NATO said in a statement.

"The service member was killed conducting train, advise, assist activities with Afghan counterparts under NATO authorities when their patrol triggered an improvised explosive device."

The Pentagon said the troop's identity would be released after family were notified.

It marks the first American combat fatality in the country since January, when a US soldier was killed while conducting a mission with Afghan special forces in Helmand's Marjah district.

"This tragic event in Helmand province reminds us that Afghanistan remains a dangerous place, and there is difficult work ahead even as Afghan forces continue to make progress in securing their own country," US Defense Secretary Ashton Carter said in a statement.

NATO said the soldier was not part of the new deployment to Lashkar Gah, which the coalition has called a "temporary effort" to train and advise local forces.

There are currently around 700 US troops in Helmand as part of a similar counter-insurgency training mission, NATO said.

Battle for Helmand province.

Fighting has recently escalated in the opium-growing province, with insurgents coming within a few kilometres of Lashkar Gah -- raising fears the city is at risk of falling.

'Serious' Taliban offensive

The turmoil convulsing Helmand, blighted by a huge opium harvest that helps fund the insurgency, underscores a rapidly unravelling security situation in Afghanistan.

Fighting has left thousands of people displaced in Helmand in recent weeks, sparking a humanitarian crisis as officials report food and water shortages.

The Taliban have also closed in on Kunduz -- the northern city they briefly seized last year in their biggest military victory so far -- leaving Afghan forces stretched on multiple fronts.

"This is a big effort by the Taliban. It's probably the most serious push we've seen of the (fighting) season," said NATO spokesman Brigadier General Charles Cleveland.

But coalition forces have insisted that neither Kunduz nor Lashkar Gah are at risk of falling to the insurgents.

NATO officially ended its combat mission in December 2014, but US forces were granted greater powers in June to strike at the insurgents as President Barack Obama vowed a more aggressive campaign.

The US intervention in Helmand has fuelled the perception that foreign powers are increasingly being drawn back into the conflict as Afghan forces struggle to rein in the Taliban.

The fighting in Helmand comes as Afghan troops are stretched on several other battlefronts across Afghanistan -- including the eastern province of Nangarhar, where the Islamic State group is making inroads.